It is required in machinery, particular that intended for commercial purposes, to indicate when the carbon brush has worn down to an extent that it shortly will be lifted off the commutator or collector ring of a motor by means of a disconnecting nipple embedded in the carbon brush. This indication takes place via a braided indicator wire which comes into conducting contact with the commutator or collector ring.
A plurality of steps are known in attempts to generate a signal only when a predetermined amount of wear has occurred. However, they basically do not provide assurance that a signal is triggered only after a previously predetermined amount of wear has taken place.
In accordance with French Patent Publication FR-A 2 223 858 it is provided to arrange a braided indicator wire in a sleeve made of an insulating material, of which at least sections extend inside an appropriately adapted recess of the carbon brush. Since the braided indicator wire itself is surrounded by an insulating material, there may be problems in arranging it exactly in the sleeve.
In accordance with EP 0 232 209 A1, optical wave guides are employed, through which optical signals are transmitted or received.
A carbon brush for collectors is known from DE 28 04 547 A1, wherein a braided indicator wire surrounded by an insulating sheath is disposed in an elongated bore. To position the braided indicator wire exactly, it is necessary to employ suitable adhesive materials. However, in case of jolts it cannot be assured that the braided indicator wire is not displaced.
To avoid these disadvantages, a pre-assembled plastic sleeve which can be inserted into a bore of a carbon brush has been provided in accordance with G 84 33 023 U1. The braided indicator wire itself is then inserted into the plastic part. The sleeve has been slitted so that the braided indicator wire remains in it. During insertion into the carbon brush, the braided indicating wire is clamped between two longitudinal halves of the sleeve which are movable in respect to each other. A large degree of exact fitting is required to assure that the braided indicating wire is clampingly received in the sleeve. Regardless of this, however, the sleeve can melt because of the high temperatures occurring in carbon brushes, so that the braided indicating wire is uncontrollably displaced.
As a remedy for this, EP 0 512 234 A2 proposes that the braided indicating wire is placed into a sleeve made of metal and is crimped together with it. This results in exact positioning of the braided indicating wire in the sleeve which, in turn, can be exactly positioned in an appropriately adapted bore of the carbon brush by means of a circumferential rim. The free end of the braided indicating wire projects past the sleeve. The free end of the braided indicating wire is at a distance from the bore in the carbon brush in which the sleeve is received. However, because of this a signal can be triggered prematurely if coal dust reaches the braided indicating wire when the bottom is being abraded.
Carbon brushes of the type mentioned at the outset can also be found in DE 41 11 206 A1 or DE-OS 21 32 053, for example. The braided indicating wires are enclosed at least on the inside or the outside in sleeves consisting of an insulating material to insulate them electrically from the carbon brush.
In accordance with U.S. Pat. No. 4,536,670 it is proposed to enclose the braided indicating wire in Teflon.RTM. to insulate it against a carbon brush. Since TEFLON is difficult to glue, it is necessary to insert the braided indicating wire twisted into a blind bore.
In accordance with U.S. Pat. No. 4,636,778, for fixing a braided indicating wire in a carbon brush the end of the braided indicating wire is inserted into a mini-carbon brush which, together with the braided indicating wire, is itself enclosed in an insulated sleeve which is glued in a blind bore of a carbon brush.